In the last year we have studied an enzyme in mammalian cells that untwists superhelical DNA. This enzyme has properties which suggest that it might serve as a swivel during DNA replication. This research program will be concerned with the purification and characterization of the enzyme, with the goals of understanding in molecular detail how it works and whether it is indeed involved in DNA replication. The DNA from a polyoma virus deletion mutant will be isolated and experiments designed to determine which of the viral gene function(s) are missing. In this regard, we will test the deletion DNA for its ability to transform a normal cell into a malignant one and its ability to replicate alone in cells. The replication of polyoma DNA will be investigated with the aim of understanding the nature of the initiation event and whether the origin for synthesis is unique or occurs at random.